1. Field of the Invention
This invention relates to the field of transfer cases for motor vehicles; more particularly, it pertains to control of a transfer case clutch.
2. Description of the Prior Art
A transfer case is a device located in a motor vehicle driveline between the output of a geared power transmission and front and rear driveshafts for transmitting power to the wheels. A transfer case may include a planetary gear set that produces both a high range, in which the transfer case output is driven at the same speed as the input, and a low range, in which the output is driven slower than its input speed. The 4xc3x972 and 4xc3x974 states of the transfer case are usually selected manually by the vehicle operator by operating a lever or switch. A first position of the lever causes a range selection device in the transfer case to direct power from the transmission output to a rear drive axle, the 4xc3x972-drive mode. A second position of the lever causes the transfer case to direct power to both a front drive axle and a rear drive axle, the 4xc3x974-drive mode.
Certain transfer case control systems either fully engage or fully disengage the secondary driveshaft and the power source. A transfer case control system for all wheel drive operation transmits power continually and variably to the front and rear driveshafts. Various techniques are available for establishing the torque split or portion of the engine torque that is transmitted to the front and rear wheels. For example, a center differential mechanism continually divides torque at a fixed ratio between the front and rear wheels perhaps 35% of torque to the front wheels and 65% to the rear wheels. But a center differential mechanism provides no variation of the torque division as needed to improve vehicle handling under certain drive conditions. A center differential usually includes a planetary gearset having an input, such as a sun gear driven by the transmission output, a first output such as a carrier connected to the rear driveshaft, and a second output such as ring gear connected to the front driveshaft.
A viscous coupling, located in parallel with the front and rear driveshafts, or the first and second outputs of a center differential, operates to mutually connect or couple the driveshafts in proportion to the speed difference between them. It produces this effect by shearing a viscous fluid located between closely spaced plates, one set of plates driven by the front driveshaft and a second set of plates driven by the rear driveshaft. Variations in the speed difference of the sets of plates increase the magnitude of the forces tending to maintain the plates at the same speed. The coupling dissipates a portion of the output power in the process of synchronizing the speeds of the front and rear driveshafts.
A hydraulically actuated clutch continually driveably connected to a primary driveshaft can be used to transmit a variable magnitude of torque to a secondary driveshaft. The magnitude of torque transmitted to the secondary shaft is controlled electronically to improve vehicle handling characteristics under certain drive conditions. Engine throttle position and the time rate of change of throttle position over the full range of vehicle speeds present a basis for improving vehicle handling through control of the clutch.
For example, when vehicle speed is high and the transmission is operating in a high gear ratio, if the vehicle operator then tips into the throttle, a low magnitude of torque at the front wheels is required to prevent or preempt wheel slip. However, when vehicle speed is low and the transmission is operating in a low gear ratio, if the vehicle operator then tips into the throttle, a relatively high magnitude of torque at the front wheels is required to prevent wheel slip.
In another example, when throttle rate is high, either due to a throttle tip-in or back-out, a relatively high magnitude of torque at the front wheels is required to prevent or preempt wheel slip. However, when the throttle rate is low, a relatively low magnitude of torque at the front wheels is required to prevent wheel slip due to the inertia of the engine and transmission. At low vehicle speed with the transmission operating in a low gear ratio, a relatively high magnitude of torque at the front wheels is required to prevent wheel slip.
There is a need, therefore, for a method, system and apparatus for controlling predictably and reliably a transfer case clutch to accommodate the particular driveline related consequences and requirements of engine throttle position, and the time rate of change of throttle position over the full range of vehicle speed to improve vehicle handling, such as the avoidance of wheel slip.
According to this invention, in a motor vehicle driveline that includes a transfer case whose output is continually connected to a first output, a clutch, operating partially engaged, responds to a control signal to change the degree of clutch engagement. A digital computer, repetitively executing a computer readable program code algorithm for operating the clutch partially engaged, selects a desired magnitude of clutch engagement with reference to functions indexed by vehicle speed and either engine throttle position or engine throttle rate. The computer repetitively updates at frequent intervals the desired degree of clutch engagement, and issues a command clutch duty cycle to a solenoid-controlled valve, which signal changes the degree of clutch engagement in response to the command signal.
The control method and system of this invention accommodates the particular driveline related requirements of engine throttle position changes, tip-ins and back-outs, and the time rate of change of throttle position over the full range of vehicle speed to improve vehicle handling, particularly the avoidance of wheel slip.
In realizing these and other advantages, a method, according to this invention, for controlling a clutch that driveably connects an input and an output with varying degrees of clutch engagement in a vehicle drive line having an engine controlled by a throttle position, includes the steps of operating the clutch partially engaged, and determining the current throttle position, throttle rate, and vehicle speed. Then both a first desired clutch engagement corresponding to the current throttle position and vehicle speed, and a second desired clutch engagement corresponding to the current throttle rate and vehicle speed are determined. The degree of clutch engagement is changed to the greater of the first desired clutch engagement and the second desired clutch engagement.
Another embodiment of this invention contemplates a method for controlling, with the aid of a digital computer, a clutch that driveably connects an input and an output with varying degrees of clutch engagement, the clutch operating in a vehicle driveline that includes an engine controlled by throttle position. The method includes first the step of inputting to and executing in the computer a computer readable program code algorithm for operating the clutch partially engaged. The computer is provided with a data base that includes at least a first desired clutch engagement that varies with the current throttle position and vehicle speed, and a second desired clutch engagement that varies with a current throttle rate and a current vehicle speed. Signals are provided at frequent intervals to the computer representing the current throttle position, and vehicle speed. The computer repetitively determines at frequent intervals during execution of the algorithm the current throttle rate, the first desired clutch engagement corresponding to the current throttle position and vehicle speed, and the second desired clutch engagement corresponding to the current throttle rate and vehicle speed. Subsequently the computer issues a command clutch duty cycle signal representing the greater of the first desired clutch engagement and the second desired clutch engagement. This command changes the current degree of clutch engagement to the degree of clutch engagement represented by the command signal.